1. Field of the Invention
This invention relates to an electric motor drive device for a camera and more particularly to a motor drive device provided with at least two gear trains for transmitting the driving force of a motor to the winding mechanism of the camera.
2. Description of the Prior Art
Generally, an electric motor drive device of the type adapted to perform camera winding by an electric motor driving operation has a DC motor rotated by the power of a battery or batteries and has a camera winding mechanism which is operated by the rotation of the motor To obtain an electric motor drive device which is small in size, light in weight and which performs suitably, it is important to select the gear ratio of the reduction gears to be used in the driving force transmission mechanism as well as the motor and the battery.
Assuming that the rotational torque of the motor is T, the rotational speed r.p.m. of the motor is N, the terminal voltage of the motor is E, the output of the motor is P, the efficiency of the motor is .eta., the current consumption by the electric motor drive device is i, the voltage of the battery is V, the internal resistance of the battery is r, the winding torque of the camera (or a loaded torque required in carrying out film winding and shutter charging) is H and the gear ratio (or a reduction ratio of the transmission gears between the motor and the winding coupler of the camera) is .epsilon., it is known that the following relation in a motor drive device of this type is obtained: ##EQU1##
In the accompany drawings, FIG. 1 shows the rotational torque T obtained from these relations. Referring to this drawing, a reference symbol T1 indicates a point at which the efficiency .eta. of the motor reaches a maximum value and T2 indicates a point at which the output P of the motor reaches a maximum value.
The open voltage V and the internal resistance r of the battery vary with the kind of battery used and/or with the amount of electric discharge W that has been effected. It is known that the electric discharge amount W can be expressed by: EQU W=(winding time).times.(current consumption i).times.number of photographed frames (6)
Meanwhile, the rotational speed (r.p.m.) of the DC motor used for such an electric motor drive device varies in proportion to the terminal voltage E. Therefore, an auxiliary power source connected in series with the power source of an electric motor drive device of this type has been proposed which has higher voltages available, as necessary, so that the winding speed can be changed by thus increasing the rotational frequency of the motor, as disclosed by U.S. Pat. No. 3,853,396 which issued on Dec. 10, 1974.
However, the rotational speed N of the motor is restricted by the mechanical strength of the motor, such as the strength of the bearing metal thereof, etc. Under a nonloaded condition, the limit of the rotational speed of a motor is generally within a range from 1000 to 15000 r.p.m. A rotational speed exceeding the limit would result in motor breakage. To solve this problem, therefore, it is necessary to use an expensive motor having a sufficiently high degree of mechanical strength. Furthermore, the arrangement of a series connected auxiliary power source to increase the rotational speed N of the motor for a higher winding speed by thus raising the terminal voltage E of the motor results in an increased rotational speed of the whole winding system. Then, another problem that arises is that the working sound becomes very noisy as the winding speed increases.
Furthermore, power sources used for electric motor drive devices of this type are selected generally from penlight batteries (size AA), a manganese type battery, and, in some cases, from penlight batteries (size AA) of the Ni-cd type. The manganese type battery (hereinafter called the manganese battery) differs from the Ni-cd type battery (hereinafter called the Ni-cd battery) in constant current discharge characteristics as shown in FIGS. 2(a) and 2(b). The manganese battery has a high internal resistance r and the voltage V and the internal resistance r vary to a great extent relative to the amount of discharge W. In addition, the discharge amount W varies with current consumption i. Therefore, when using the manganese battery, it is preferable to have the current consumption as low as possible within the necessary range of current consumption. When using the Ni-cd battery, the internal resistance is low and the voltage V and internal resistance vary to a lesser extent in relation to the amount of discharge W. It is known that, compared with the manganese battery, the Ni-cd battery allows a higher degree of current consumption i.
Accordingly, the manganese and Ni-cd batteries, which thus differ in characteristics, are used together for an electric motor drive device since the manganese battery can be readily replaced and the Ni-cd battery excels in performance even though it takes time for charging, etc. When the characteristic (or the gear ratio .epsilon.) of the electric motor drive device is set to suit one of the batteries, the conditions required for appropriate use of the other cannot be satisfied. Under these conditions, there arises the following problem:
Generally, in an electric motor drive device, in order to ensure a certain degree of efficiency (or the amount of film to be fed), the rotational torque T must be set between a maximum efficiency point T1 and a maximum output point T2. However, as shown in FIGS. 3(a) and 3(b), the maximum efficiency point T11 and the maximum output point T21 obtainable with the manganese battery are lower than those (T12 and T22) obtainable with the Ni-cd battery. Therefore, when the rotational torque T is set at a value suitable for the manganese battery, use of the Ni-cd battery results in a much lowered output P. Then, the winding speed becomes insufficient. Furthermore, when the rotational torque T is set at a value suitable for the Ni-cd battery, use of the manganese battery results in a much lowered efficiency .eta.. The lowered efficiency causes a great reduction in the amount of film which is fed.
The rotational torque T of the motor is determined from the gear ratio .epsilon. through Formula (2) above if the winding torque H of the camera is unvarying. In the conventional electric motor drive device having a fixed gear ratio, therefore, it has been extremely difficult to obtain a certain acceptable winding speed and a certain acceptable amount of film feeding when both the manganese battery and Ni-cd battery are used.
The present invention is directed to the solution of the above-mentioned problem encountered when using conventional electric motor drive devices.
It is, therefore, an object of the invention to provide an electric motor drive device in which the gear ratio of the transmission gear arrangement is shiftable according to the conditions of the batteries to be used, so that the winding action of the camera can be carried out in an optimum condition according to the number of batteries and the particular kinds to be used.
It is another object of the invention to provide an electric motor drive device which shifts the gear ratio of the transmission gears by use of an auxiliary power source in such a manner that the winding speed of the camera can be increased without increasing the rotational speed of the motor.
It is a further object of the invention to provide an electric motor drive device in which the gear ratio of transmission gears is shiftable depending on the kind of batteries used, so that the winding speed and amount of film fed will not be lowered by the use of different kinds of batteries.
These and further objects, features and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.